-t or -ed in past participle?

Once upon a time, several English past participles that now end in -ed used to end in -t

So we had: I kist a girl (and I liked it). I dropt the glass and curst my clumsiness. The referee stopt the fight.

Those are all gone now, but a few brave verbs hung on to the -t ending: crept, dealt, felt, kept, left, meant.

(By the way, these verbs are also called "strong" verbs, which is another way of saying "irregular". They are "strong" because,  like Chuck Norris, they set their own rules).

Other verbs just couldn't make up their minds, and got stuck between regular and irregular (or "weak" and "strong", if you prefer) -- So you don't have to make up your mind either:

Bereaved or bereft
Burned or burnt
Dreamed or dreamt  (the -t ending looks more British to me)
Kneeled or knelt (the -ed looks a bit weird, but that might just be me)
Leaned or leant (the -t ending looks more British to me)
Learned or learnt (the -t ending looks more British to me)
Spoiled or spoilt
Smelled or smelt
Spelled or spelt
Spilled or spilt.

It's hard to measure the popularity in print of these. My impression is that -ed is winning. You can check Google's ngram to see  for yourself. Here, for example, is a graph showing the increasing popularity in print of spelled. 

 Ironic, isn't it, that it's difficult to spell the past tense of spell?

Quick rule: If in doubt, go for the -ed ending.